Have you ever had to stand shivering in the cold morning air waiting for hot water with your arm outstretched, fingers extended into the cold stream of water. It seems to take forever for the water to get hot. Speed up it up with a circulating system.
Traditionally, if you had a circulating system, it meant that you had a hot water pipe that ran in a big loop from the outlet of the heater to each fixture one after the other, and then back to the inlet of the water heater through a small pump. By continuously pumping water through the loop you have instant hot water at every fixture.
This is very convenient. Hotels and motels have circulating systems; otherwise it could take hours to get hot water with such long pipes as they have.
However, instant hot water is also very expensive. Not only do you have to pay for the energy to run the pump continuously for long periods of time or even full time, but you also have to pay for the heat energy being radiated into the environment from that big loop of piping. The cost of heating the water is considerably more than the cost to run the pump.
There are a number of manufacturers producing circulating systems that utilize the cold water line as the return line.
Some of these pumping systems use small pumps that pump water very slowly, and are temperature controlled. The pump turns on when the water temperature drops below a set point, and then shuts off when the water temperature in the pipe reaches another set point. This keeps the water your entire piping system luke-warm . The water is warmer near the heater and colder the further you get from the heater, but it is tepid through out the piping system.
Most folks don't really want the cold pipe full of warm or tepid water. You don't get "Instant Hot Water" as the manufacturers claim. I think they should be honest and call them "Instant Tepid Water Systems". The Lang Auto-Circ and The Grundfos Comfort System are two such systems, and the Hot Water Lobster is another. The Hot Water Lobster doesn't have a pump, but relies on the fact that heated water rises, and the Lobster valve has to be substantially higher than the heater for the system to work.
Unfortunately these systems consume much more energy since you are heating that big loop of piping and return line and its surroundings. The cost of the energy will far outweigh any monetary savings from the water conservation aspect.
Another class of systems that use the cold water line as the return line is the "Hot Water Demand Systems". These systems only pump the water to the fixture when hot water is "demanded" by the user. The pumping costs are small; typically demand systems use less than .00 per year in electricity costs. This is because they run for such a brief time, typically less than 1 minute per use.
When the user turns on the pump it runs until hot water reaches the fixture and then the pump automatically shuts off. The cold water line does not end up full of warm water.
With the demand system the energy consumed is no more than if the user just ran the tap as normal. And since the water is not being circulated it does not affect the life of your tankless heater or the warranty.
The water is pumped more quickly than if you ran the faucet full throttle. You save time, water, energy, and money!
There are at least three manufacturers of demand systems, Metlund, RedyTemp, and Chilipepper. The Chilipepper will run any tankless water heater. Some models of the Metlund system will run tankless water heaters, and some won't, so be sure to inquire before you make a purchase.
